Noise emission from technical installations, particularly power systems, is a safety and environmental concern. Legislation dictates the admissible sound pressure level that a noise source in a certain location may continuously make so that reduced noise emission is a key requirement for low impact environmental integration. Regulations governing these levels vary currently from country to country. In Europe, European Directives 2000-194-EC and 2002-49-EC provide standards. In Germany, the standard values are based on VDI standard 2058, and were adopted by the Technical Directive on Noise Abatement prescribed by law. The maximum allowable values depend on the character of the surroundings and the time of day. For example, 65 dB(A) are allowed in a prevailing industrial surrounding during the day, whereas only 35 dB(A) are allowed in exclusively residential surroundings during the night. These regulations define environmental requirements and acoustical targets for wind turbine installations.
Noise generated by wind turbines is partly mechanical, and partly aerodynamic. Mechanical noise is generated mainly from machinery in the nacelle, particularly the gearbox and the generator, although there may also be contributions from cooling fans, auxiliary equipment (such as pumps and compressors), bearings and the yaw system. Machinery noise is related to high frequency dynamic forces than can generate structure borne or air-borne mechanical noise. Therein, the low frequency noise associated with high forces can be assigned to static and quasi-static loads, e.g. the rotor power torque caused by the wind. Quasi-static structural dynamics being typically below 20 Hz does not relate to noise generation but structural fatigue. High frequency structure-borne mechanical noise, typically in the frequency range of 20-1000 Hz is often associated to dynamic forces related to gear pair meshing mechanical forces from mechanical gear systems or stator-rotor pole meshing electrostatic forces from electrical generator systems. Because dynamic forces from machinery correspond to a discrete force spectrum described by meshing frequencies, the noise spectrum related can also be discrete in nature and result in tonal noise components.
Structure-borne mechanical noise is therefore initiated by dynamic forces from machinery where these dynamic forces are injected by mechanical gear or generator systems at machinery mounting interfaces into the supporting structures or chassis. Transferred dynamic forces through chassis response can then excite structural vibration modes from larger structures such as tower, blades and nacelle enclosure. By coupling with surrounding air, the vibrating surfaces generate sound radiation. Force transfer and noise radiation are especially effective when structural resonances are matching along the transfer chain and when the structural modes wavelength matches the acoustic wavelength in air related to acoustic radiation maxima.
Therefore, damping or isolation of the machinery vibrations are desirable. Another attempt is described in DE 199 30 751 A1 by F. Mitsch filed Jul. 2, 1999, disclosing a method for reducing vibrations of components of a wind turbine. According to said method, a plurality of bearings made of a very soft elastomeric material are used for damping the vibrations. Also, cylindrical elastomeric bushings are used for vibration isolation in existing wind turbines. However, cylindrical elastomer bushings have several disadvantages, namely that they are highly non-linear elements and, therefore, become dynamically hard as steel if high loads are applied. Furthermore, elastomer bushings exhibit poor vibration isolation over the entire frequency range due to their cylindrical shape and their dynamic stiffness increase with frequency. Moreover, the vibration isolation ability of elastomer bushings rapidly decreases with environmental temperature. A further drawback of standard cylindrical elastomer bushings is that they only operate providing low dynamic stiffness and hence some vibration isolation potential along the radial axis direction and allow only very small lateral displacement, thus enabling only very low isolation on the other axis because of typically high radial dynamic stiffness incompatible with vibration isolation.
An additional aspect of vibration isolation is that the machinery support must be able to withstand high loads caused by high wind speeds or wind gusts. Therefore, a conflict exists between vibration isolation requiring very soft mounts to provide vibration isolation for noise control purpose and very hard mounts that can sustain high loads and allow only small displacements of the machinery.
It is therefore an object of the present invention to provide a vibration isolating suspension system that overcomes the above mentioned problems associated with the prior art at least partially. Furthermore, it is an object of the present invention to provide a wind turbine with an improved suspension system.